Lock some directions (X,Y,Z) of transformation (scale, rotation, location) in edit mode?
While in edit mode, with elements (faces, edges or vertices) selected, you can cycle through various translate/scaling/rotation restrictions by hitting the X,Y or Z buttons. While moving (hit 'g') a vertex (for example) with your mouse, hit 'X' once. The vertex will be restricted to moving along the world coordinate x-axis from its original location. Hit 'X' again, and it is restricted to the local coordinate x-axis. Hit 'X' once again, and the restriction is removed.

You may also use Shift with X,Y or Z. This would place restrictions on ONLY the selected axis. For example, 'X' allows ONLY changes along the x axis, while 'Shift-X' allows changes on the y and z axis but NOT the x axis.

Again, this applies equally to Moving (g), Rotating (r) and Scaling (s).